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The observed absolute luminosity of any stellar system is an indication of the approximate amount of luminous matter in such a system. In order to derive trustworthy values of the masses of nebulae from their absolute luminosities, however, detailed information on the following three points is necessary.

1. According to the mass-luminosity relation, the conversion factor from absolute luminosity to mass is different for different types of stars. The same holds true for any kind of luminous matter. In order to determine the conversion factor for a nebula as a whole, we must know, therefore, in what proportions all the possible luminous components are represented in this nebula.

2. We must know how much dark matter is incorporated in nebulae in the form of cool and cold stars, macroscopic and microscopic solid bodies, and gases.

3. Finally, we must know to what extent the apparent luminosity of a given nebula is diminished by the internal absorption of radiation because of the presence of dark matter.

Data are meager (1) on point 1. Accurate information on points 2 and 3 is almost entirely lacking. Estimates of the masses of nebulae from their observed luminosities are therefore incomplete and can at best furnish only the lowest limits for the values of these masses.

1 It should, however, be mentioned that certain spiral nebulae seem to be stellar systems similar in composition to the local Kapteyn system of our galaxy. For such systems the conversion factors may with some confidence be set equal to the conversion factor of the Kapteyn system. See also E. Hubble, Ap. J., 64, 148, 1929. Back.

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